mktemp within apps-addupdate doesn't support older versions of OS X

[johnfonner]

apps-addupdate now uses a mktemp command with no arguments to create a temporary file. Older versions of OS X (and maybe other OSes) require a template for mktemp.

According to https://stackoverflow.com/questions/31396985/why-is-mktemp-on-os-x-broken-with-a-command-that-worked-on-linux, the most OS agnostic way of call mktemp is:

mktemp "${TMPDIR:-/tmp}/XXXXXXXXX"